The present invention broadly relates to cutting tools and, more specifically, pertains to a new and improved construction of a cutting tool for planing or shaping the flanks of hardened gear teeth.
Generally speaking, the cutting tool of the present invention for planing or shaping gear tooth flanks on a roughed-out or pre-formed gear blank or workpiece is of the type comprising a cutting tool insert holder and a plurality of cutting tool inserts fastened to the holder.
More specifically, the invention relates to a cutting tool of the aforementioned type in which the cutting tool inserts are formed according to the commonly, assigned related U.S. patent application Ser. No. 06/459,358, filed by Bloch et al on Jan. 19, 1983. These cutting tool inserts comprise a base or support layer of hard metal and a sintered surface or cover layer of very hard polycrystalline cutting material and are oriented in relation to the cutting direction such that the surface of the surface or cover layer forms a free or clearance surface and only one of the side surfaces of the surface or cover layer lies in the cutting surface or plane.
Cutting tools employing cutting tool inserts made according to the aforementioned patent application have proven to be exceptionally good; in particular, they have made it possible to plane or machine hardened gear tooth flanks for the first time. The cutting tool inserts have an unexpectedly long service time between rotation or exchange, so that even gears with a large number of teeth can usually be planed or shaped in a single operation without having to turn the cutting tool inserts or exchange them. Problems have, however, arisen in planing gear teeth having a module greater than 8 since the only cutting tool inserts available up to now are too small to be able to plane gear teeth having a module greater than 8 in hobbing procedure.
Applicant has therefore produced cutting tools of the aforementioned type in which a plurality of cutting tool inserts are fastened to the cutting tool insert holder in adjacent abutting relationship without interstitial gaps and with their active primary cutting edges aligned. In this way, a cutting tool is obtained whose total cutting edge length is sufficient to hob gear tooth flanks having a module greater than 8.
The machining results obtained with these tools are, however, not completely satisfactory, since the cutting tool inserts, in spite of the joint between neighboring cutting tool inserts being made as near as possible to zero width, tend to chip or break off in the region of those joints sooner or later. Even chips which are hardly visible to the naked eye can increase the cutting pressure or force to such an extent that the cutting tool insert holder or the gear tooth which it is cutting, or both, elastically deform and give rise to inaccuracies. These inaccuracies render the gear teeth hobbed in this manner unusable without further processing. They require the provision of a greater grinding allowance for subsequent smooth or finish grinding.
Due to the nevertheless considerable durability of the cutting tool inserts, their employment in the place of rough grinding can still be economical for gear teeth having a module greater than 8 even when the cutting tool has to be rotated, sharpened or exchanged during a revolution of the workpiece. It is however difficult and time-consuming to adjust the rotated, sharpened or exchanged cutting tool to a sufficient degree of precision that the machined accuracy of the workpiece does not suffer due to the change. For this reason as well, an increased grinding allowance has heretofore had to be made for the subsequent finish grinding.